The present invention relates generally to a microstrip termination. More particularly, the present invention relates to a microstrip termination using a grounded thin film resistor.
Terminations are common components in most microwave systems. Microstrip terminations are easy to process using thin film technology, but the performance drops off rapidly with increasing frequency. Thin film technology typically uses an alumina substrate, with gold and resistor material sputtered onto the substrate, which is then patterned with photolithography techniques to define transmission line traces and resistors. Thick films could also be used, but they typically do not go to high frequencies (above 20 Ghz).
FIG. 1 illustrates a standard microstrip termination, known as an edge ground circuit. In the microstrip 100 of FIG. 1, a microstrip transmission line 104, typically a metal line, is formed on the microstrip substrate 102, made of a dielectric such as alumina. An area of resistive material 106 is formed on the substrate 102 along the transmission line 104 near an edge ground. The edge ground is formed with a transmission line 110 connecting the resistive material 106 to the metal plated edge 108 which connects to a metal ground plane 112 deposited on the bottom surface of the substrate below the trace and resistive material. The resistive material 106 is used to terminate a signal propagating along the transmission line by matching the impedance of the transmission line and preventing reflection of the propagating signal.
FIG. 2 illustrates another standard microstrip termination used when a termination is required away from an edge. This termination 200 also includes a microstrip substrate 202 typically having a metal bottom layer 212, a transmission line 204, and an area of thin film resistive material 206. The substrate 202 also has an area of metal 208 between the resistive material 206 and the edge of the substrate 202. The substrate 202 typically contains Monolithic Microwave Integrated Circuits (MMICs) connected to the transmission line 204, and the substrate 202 is mounted on a carrier. A carrier is typically a thin metal plate, on the order of xc2xd to 1 mm thick, and provides the ground for the microstrip substrate and the MMICs thereon in addition to the metal bottom layer 212.
The termination of FIG. 2 further uses a ground via 210. The via 210 is formed from metal deposited in a hole in the substrate that extends from the area of metal 208 on the top surface of the substrate to the metal bottom layer 212. The termination shown in FIG. 2 can be placed anywhere in a subsystem circuit, but the performance is generally worse than the edge ground circuit of FIG. 1. The poor performance is due to the increased inductance to ground resulting from the small via.
FIG. 3 shows the typical performance of an edge grounded microstrip termination. One reason for the poor performance illustrated in the figure is that the xe2x80x9cenvironmentxe2x80x9d of the resistor is not correct.
It is desired that a DC to microwave termination be a reflectionless transition from the transmission line impedance of Zo to ground. The reactive part of the transition should, therefore, match the resistive part from the Zo ohm line to ground. For instance, if the midway resistance of a 50 ohm termination resistor is 25 ohms, the surrounding reactive environment is preferably also 25 ohms. In a coaxial termination, the outer conductor diameter over the resistor is tapered down from the 50 ohm diameter to the diameter of the resistor at the ground end, to provide a smooth impedance transition xe2x80x9cenvironment.xe2x80x9d It is therefore desirable to provide a microstrip termination that sufficiently prevents signal reflection by providing a smooth impedance transition.
The present invention provides a microstrip substrate that has a tapered edge used to form a tapered impedance ground plane, which provides a smooth impedance transition to ground for a termination. To form the termination, the substrate has a transmission line on the top surface and a metal ground layer on the bottom surface. An area of thin film resistive material is positioned on the top surface between the transmission line and the tapered ground plane. A layer of conductive material extends from the top surface across the tapered edge to the bottom metal ground layer. The tapered ground plane is at an angle with respect to the top surface of the substrate to provide a smooth change in impedance for a signal propagating through the thin film resistor.